I have never gone, but one of my friends used to go all the time until he tweaked his knee and tore 3 out of 4 tendons, and dislocated his knee. This happened 2 years ago, and he is still hobbling around on a brace. They even had to take a tendon from a cadaver to replace his.
The funny thing is that he had a good amount of experience, and did this just going down a hill. No stunts or anything foolish.